it isn't how i define existentialism that matters. it is how the existentialists define the term. i suggest you read the book mentioned above to get a true grasp of it, along with such things as Being and Nothingness and Nausea by Sartre, The Stranger and The Fall by Camus.
however, if you're looking for a quick lesson, i can try and paraphrase some of Sartre's examples for you. here we go:
the basic definition is "existence before essence", meaning, man is born empty and his actions from that point on define the man that he is. furthermore, action is all that defines him. the best example that Sartre gives of this goes something like this: when you call someone a coward, it is because they have chosen to act cowardly. they were not a coward when they were born and they were not a coward up until the point that they acted as one. thus, they are responsible for their own cowardice, just as everyone is responsible for who they are.
i hope this helped. as i said, i am not an expert and i urge you to read more from those who are.